Two-tube shock absorber

ABSTRACT

A two-tube shock absorber for automotive vehicles is furnished with a shock absorber valve positioned at the lower end of its power cylinder and comprising a pilot control stage, a main stage to adjust its &#34;soft&#34; characteristic curve which is desired for reasons of driving comfort, or a special characteristic curve out of a field of potential characteristic curves. According to the invention, in a lower range of piston speed the shock absorbing power is determined exclusively by the variable cross-sectional area of flow of the pilot control stage wherein the main stage remains closed. In an upper range of piston speed the shock absorbing power is determined by the variable flow cross-sectional areas both of the main stage and of the pilot control stage.

TECHNICAL FIELD

The invention is related to a controllable two-tube shock absorber withcontrollable variable shock absorbing power for automotive vehicles, andmore particularly relates to shock absorbers comprised of a powercylinder wherein the interior space is subdivided into a first and asecond power chambers by means of a piston slidable by a piston rod;storage tank partly filled with oil, wherein hydraulic connectionsafford a compensation of volume between the power chambers and thestorage tank.

BACKGROUND OF THE INVENTION

A two-tube shock absorber of this kind is known from the internationalpatent application No. WO 89/09891. A particular feature of this priorart two-tube shock absorber is its shock absorbing power that ispredetermined exclusively by means of a two-stage controllable shockabsorber valve which is disposed at the lower end of the central tubeand through which the fluid flows in one direction only. With thisarrangement, the pilot control stage of the shock absorber valve servesexclusively to control the main stage wherein the flow cross-sectionalarea determines the variance of absorbing power.

In the state of the art controllable shock absorber however, adisadvantage is that beyond the electromagnetic actuating power at thepilot control stage a minimum hydraulic pressure is required in thesystem in order to safeguard the functioning of the shock absorber inthe customary range of application. Furthermore, the control times whichare achievable by this arrangement and which are too long for aquick-acting shock absorber system are less advantageous. The "soft"characteristic and the quick adjustability, which are desired fordriving comfort, cannot be attained for the aforementioned reasons.

It is, therefore, the object of the present invention to provide atwo-tube shock absorber of the kind discussed which overcomes theaforementioned disadvantages. Moreover, a two-tube shock absorber isprovided which affords the adjustment of a special desiredcharacteristic, wherein the shock absorbing power is dependent on thepiston speed, out of a field of potential characteristic curves.

According to the invention this object is achieved in that in a lowerrange of piston speed the shock absorbing power is exclusivelydetermined by the variable flow cross-sectional area of the pilotcontrol stage, while the main stage remains closed, and in that in anupper range of piston speed the shock absorbing power is determined bythe variable flow cross-sectional areas both in the main stage and inthe pilot control stage.

With this arrangement, it is of advantage that in the upper range ofpiston speed the main stage follows the position of the pilot controlstage. Due to this provision, a more elevated dynamic ratio of the valveadjustment jointly with a simultaneous reduction of the vibrationtendency of the shock absorber, particularly when utilized in a movinggear control process, is achieved.

A further advantage of this invention is that the main stage as well asthe pilot control stage are arranged so as to be separate from thesecond non-return valve in a valve housing, wherein the main stageincludes a main slide valve interacting with a control edge which isconfigured within the valve housing. The pilot control stage includes anelectromagnetically actuatable control slide valve interacting withcontrol bores which are provided in the main slide valve. A shockabsorber valve having this set-up and presenting a two-stageconfiguration requires less actuating capacity, and affords a betterhandling of more elevated absorbing power levels.

The control edge is preferably positioned in proximity to the flow-offbores which are configured in the valve housing and which are used inconnection with the storage tank. The flow-off bore's size and geometricshape influence the performance characteristics of the main stage. Inits upper part, the main slide valve is configured open and is furnishedwith slide valve bores which allow the bores to be brought intoconnection with the flow-off bores.

According to another preferred embodiment of the invention, the mainslide valve is prestressed in the closing direction of the main stage bymeans of an optimum first compression spring. The main slide valve is inabutment against a stop. The first compression spring assists theclosing of the main stage.

Any suction or displacement problems at the main slide valve which occurduring the operation of the inventive shock absorber are preferablyeliminated if the abutment of the main slide valve at the stop includesaxial projections.

A further advantage of the invention is that the main slide valve isprovided with restricting bores which end up in a hydraulic chamberdefined within the valve housing by the main slide valve. Due to thisprovision, both a smooth oil supply for the pilot control stage isattained and pressure drops in the event of the passage through saidrestricting bores is attained, which are of vital importance for regularfunctioning.

An advantageous coupling of the main stage to the pilot control stage isattained in another embodiment of the invention. The control slide valveis guided in the main slide valve and may be abutted against it underthe prestressing action by a second compression spring.

A particularly advantageous embodiment of the invention includes a pilotcontrol stage which is electromagnetically actuatable by a plunger coilinteracting with a permanent magnet. This embodiment comprises a controlslide valve captivated to the plunger coil support by means of acaptivating element wherein the captivating element head may be inabutment against the main slide valve. Due to this provision, an exactcontrol of the absorbing power in the lower range of the piston speedand an adjustability of the shock absorber valve and, as a consequence,variations of the absorbing power are rendered possible. Simultaneously,a swinging of the control slide valve over the control bores isprevented. In this embodiment, the favorable dynamic behavior of theplunger coil is of particular advantage.

According to another preferred embodiment, the actuating unit iscomprised of the plunger coil and of the permanent magnet positioned ina hydraulic chamber which is in connection with the storage tank, sothat an end of the captivating element, which is fixed to the plungercoil support, is subject to the action of the pressure existing withinthe storage tank. The aim achieved is that the plunger coil does notundergo the pressure which exists in the storage tank. Simultaneously,the effect of a downwardly directed force is applied on the captivatingelement.

In the event of a failure or of faulty control of the shock absorbervalve to prevent an inadmissibly elevated absorbing power in thepressure stage, a non-controllable valve is provided along the lines ofthe invention parallel to the main stage, or the pilot control stage,which is active exclusively in the pressure stage of the shock absorber.

In this embodiment, the valve is preferably configured in the range ofthe second non-return valve, and comprised of a ball interacting with avalve seat and being prestressed by a spring, and, as already discussedabove, becomes effective exclusively in the pressure stage.

This embodiment will be of further advantage when the valve is arrangedin a connection between the second chamber and the storage tank which isformed by bores in a disc-shaped part of the second non-return valve. Inthe valve housing, the bore which is provided aligns with one of theflow-off bores. The aim achieved by this inventive provision is that thevolumetric stream flowing through the valve is led into the alreadyexisting storage tank, so that no additional oil reservoir will benecessary.

According to a further advantage of the invention, the electromagneticactuation of the pilot control stage takes place by means of aproportional magnet whose armature is connected to the control slidevalve by means of a captivating element, wherein the head of thecaptivating element is abutted against the main slide valve. Theadvantage of this arrangement is seen in the favorable ratio between themaximum magnetic force Fmax supplied and the required overall space, orthe required mass.

According to another preferred embodiment of the invention, the mainstage as well as the pilot control stage are arranged within a valvehousing so as to be separate from the second non-return valve. The mainstage includes a valve piston interacting with a sealing seat which isconfigured as a disc-shaped element of the second non-return valve. Thepilot control stage includes an electromagnetically actuatable controlslide valve interacting with control bores which are configured in thevalve piston. The use of a seat valve having such a set-up aids inovercoming soiling problems which occur in the system.

Furthermore, it will be of advantage when the disc-shaped elementjointly with the valve housing defines a hydraulic annular chamber intowhich the valve piston projects and which is in connection with thestorage tank. The above-mentioned control bores ends up in the annularchamber. These provisions provide a simple way to combine the main stagestream and the pilot control stage stream and to convey them into acommon storage tank.

Other advantages of the invention are that the main stage as well as thepilot control stage are positioned within a valve housing so as to beseparate from the second non-return valve. The main stage includes apiston which is sealedly guided within the valve housing, which bears avalve closing plate and which is furnished with an axial bore. The valvehousing includes a sealing seat which allows the main stage to be shutoff and released by the valve closing plate and which is configured as adisc-shaped element of the second non-return valve. The pilot controlstage includes a cylindrical chamber configured in the valve housing.The cylindrical chamber accommodates a control slide valve elasticallyprestressed in the closing direction. An axially extending cylindricalwall is provided with radial bores which interact with control boresconfigured in the control slide valve and which afford a hydraulic linkbetween a chamber being defined by the piston within the valve housing,on one hand, and the storage tank, on the other hand. This arrangementallows a flow through the pilot control stage at a right angle to thedirection of movement of the control slide valve, even when the valve iswide open, since the position of the pilot control stage is influencedonly to a slight extent.

When the pilot control stage is actuatable electromagnetically, and thecontrol slide valve is configured with the support of a plunger coilwhich comprises part of the electromagnetic actuating device, a directcontrol of the shock absorber valve in the lower range of piston speedwill be attained.

An increase of the quality of the control process is achieved in apreferred embodiment wherein a wall is provided which defines thecylindrical chamber. The wall extends in horizontal direction andincludes a stepped bore wherein a section having a smaller diameter isin the chamber and wherein a section having a larger diameteraccommodates an axially slidable thrust member which interacts with thefront face of the control slide valve.

As an alternative, an intermediate position of the control slide valvein the condition of rest may be provided in which a partial connectionis established between the chamber which is defined within the valvehousing by the main slide valve, or by the valve piston, on one hand,and the storage tank, on the other hand. The intermediate position ofthe control slide valve is achieved by a third compression spring whichcounteracts the second compression spring prestressing the control slidevalve.

A compact design is, finally, achieved in the aforementioned two-tubeshock absorber in that the third compression spring is supported at thehorizontal wall defining the cylindrical chamber.

In a further feature of the invention, an electromagnetic actuating unitfor the shock absorber valve of the inventive controllable two-tubevibration absorber is disclosed which comprises a closed magnet framewhich accommodates a permanent magnet as well as an electric coilinteracting with the permanent magnet. In order to achieve a perfectmagnetic screening effect, the range of the magnet frame carrying themagnetic flux is configured in the shape of a hollow ring in which thepermanent magnet is disposed in such a way that its magnetic power fluxis split into two partial fluxes directed opposite to each other. Inthis manner, the electromagnetic system is very effectively screened offoutside and inside so that the interior space of the magnet frame isfree of magnetic field of force. The interior space may thereby be usedfor the incorporation of components of the shock absorber valve,components of sensors and other components. The purpose achieved by thedivision of the power flux is that the wall thicknesses of the groundingmeans constituted by the magnet frame may be considerably reduced.

An advantage of the inventive actuating unit is a coil configured as aplunger coil, wherein a coil support is guided in guide slots providedin the lower part of the magnet frame and is positioned outside themagnet. The coil support is connected to a coil form bearing the windingof the electric coil by means of guide pins which extend throughopenings in the magnet frame. In this configuration, the plunger coilmay be radially encircled by the permanent magnet or may radiallyencircle the permanent magnet.

In a further embodiment of the invention, ring-shaped supportingelements formed of magnetically non-conductive material fix thepermanent magnet in position.

In another embodiment of the inventive actuating unit, the electric coilis unmovably positioned in the magnet frame, and the permanent magnet isfixed to the wall range of the magnet frame which radially internallydefines the hollow ring and is axially movably guided in guides ofmagnetically non-conductive material.

In another embodiment of the actuating unit, a safe guidance of themoving coil is achieved in that the coil support of the plunger coilcomprises a cylindrical range which is guided in the wall range of themagnet frame radially internally defining the hollow ring.

Another embodiment of the invention operates with a magnetic field offorce comparable to the radially magnetized ring-shaped permanentmagnet. In this embodiment, the wall range radially internally definingthe hollow ring is configured in the shape of a polygon, wherein thefront surfaces are axially magnetizable permanent magnets. The permanentmagnets are cuboid-shaped, circular disc-shaped or ring segment-shaped.

An especially compact embodiment of the inventive actuating unit isdistinguished in that the electric coil is formed by a single-layerwinding of metal ribbon. In this embodiment, the metal ribbon maypreferably be comprised of a unilaterally insulated strip of copper foilor of a superficially oxidized strip of aluminum foil.

Further details, features and advantages of the invention will berevealed by the following description of six embodiments, makingreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a first preferred embodiment of a two-tube shock absorberwith variable absorbing power according to the invention in a sectionedrepresentation.

FIG. 2 shows a cross section of the lower range of the two-tube shockabsorber illustrated in FIG. 1 with a shock absorber valve in anupscaled representation.

FIG. 3 shows a second embodiment of the invention in a cross-sectionalrepresentation corresponding to FIG. 2.

FIG. 4 shows a third embodiment of the invention in a cross-sectionalrepresentation corresponding to FIG. 2.

FIG. 5 shows a fourth embodiment of the invention which is slightlymodified with respect to the first embodiment illustrated in FIG. 1, ina cross-sectional representation corresponding to FIG. 2.

FIG. 6 shows a fifth preferred embodiment of the inventive two-tubeshock absorber in a cross-sectional representation corresponding to FIG.2.

FIG. 7 shows a sixth preferred embodiment of the inventive two-tubeshock absorber in a cross-sectional representation corresponding to FIG.2.

FIG. 8 shows a second embodiment of the electromagnetic actuating unitof the shock absorber valve used in the inventive two-tube shockabsorber.

FIG. 9 shows a third embodiment of the electromagnetic actuating unit ofthe shock absorber valve used in the inventive two-tube shock absorber.

FIG. 10 shows a fourth embodiment of the electromagnetic actuating unitof the shock absorber valve used in the inventive two-tube shockabsorber.

FIG. 11 shows a fifth embodiment of the electromagnetic actuating unitof the shock absorber valve used in the inventive two-tube shockabsorber.

FIG. 12 shows a sixth embodiment of the electromagnetic actuating unitof the shock absorber valve used in the inventive two-tube shockabsorber.

FIG. 13 shows a further embodiment of the electric coil used in theelectromagnetic actuating unit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The two-tube shock absorber according to the invention which isillustrated in FIGS. 1 and 2 is furnished with a hollow power cylinder 1and with an external tube 7 positioned coaxially with said powercylinder 1, so that a storage tank 8 having an annulus-shaped crosssection and being partly filled with oil is formed between the powercylinder 1 and external tube 7. The interior space of the power cylinder1 is subdivided by a piston 3 slidable by a tubular piston rod 2 into afirst power chamber 5, which is configured above said piston 3, and asecond power chamber 6, which is configured below said piston 3. In thisconfiguration, the piston 3 is provided with a first non-return valve 4whose function will be explained in the following text. In its centralrange piston 3 is penetrated by a central tube 9 which projects into thepiston rod 2 and which during the operation of the shock absorberaffords a compensation of volume between first power chamber 5 andstorage tank 8, or between second power chamber 6 and storage tank 8.

At the lower end of the central tube 9, a valve arrangement is formed bya second non-return valve 13 and a shock absorber valve 10 of apreferably two-stage configuration and which serves for the variation ofthe flow cross-sectional area of the connection between central tube 9and the storage tank 8. The shock absorber valve 10 is comprised of apreferably electromagnetically actuatable pilot control stage 11 and amain stage 12 which follows the pilot control stage 11 in the upperrange of piston speed. In this configuration, the second non-returnvalve 13 comprises a disc-shaped element 38 which is positioned at thelower end of the power cylinder 1 and in which axially extending bores,shown as ducts 61, are provided which interact with a spring disc 62enabling their closing or opening. Disc-shaped element 38 defines ahydraulic annular chamber 44 which is in connection with the storagetank 8.

The shock absorber valve 10 comprises a valve housing 14 which is inabutment in an axial direction against the disc-shaped element 38 andwhich accommodates both the pilot control stage 11 and the main stage12. The main stage 12 includes a main slide valve 16 which is slidinglyguided within the valve housing 14 and which interacts with a controledge 15 configured in the valve housing 14. In this configuration, thecontrol edge 15 is located in proximity to flow-off bores 19 which arein connection with the storage tank 8. In its upper part, the main slidevalve 16 is configured open and furnished with a plurality of radialbores 20 which are uniformly distributed over its circumference andwhich are brought into connection with flow-off bores 19 when the mainstage 12 opens. In its position of rest, said main slide valve 16 is inabutment against the end of the central tube 9 under the action of afirst compression spring 21 and by means of axial projections 22.

A hydraulic chamber 24 is defined within valve housing 14 which is inconnection with the interior space of the central tube 9 throughrestricting bores 23. Hydraulic chamber 24 is defined in downwarddirection by a control slide valve 18 which is axially slidingly guidedwithin the main slide valve 16. Hydraulic chamber 24 interacts withcontrol bores 17 configured within the main slide valve 16 in proximityto the flow-off bores 19. Control bores 17 extend in a radial direction.Control slide valve 18 is abutted against the main slide valve 16 underthe prestressing action by a second compression spring 25 which ispositioned coaxially with the first compression spring 21.

Control slide valve 18 is actuatable by means of an electromagneticactuating unit which is comprised of a plunger coil 26 and of apermanent magnet 30 which are accommodated in a magnet frame 69 ofmagnetically conductive material, such as iron. In this embodiment,magnet frame 69 comprises a cylindrical center range 76 of the valvehousing 14 and of a cup-shaped screening cage 77. The magnetic flux offorce-carrying range of magnet frame 69 is designed as a hollow ring, sothat the magnetic flux of force of the radially polarized permanentmagnet 30 which is positioned therein is subdivided into two part-fluxes80 extending in opposite directions to each other (see FIG. 8). Due tothese provisions, the wall thickness of the components 76, 77 may bedimensioned only for half the flux density.

As is seen in FIG. 2, the cylindrical center range 76 of the valvehousing 14 is furnished in its lower portion with guide slots 70 whichserve to guide a plunger coil support 27 which consists of magneticallynon-conductive material. In this configuration, the ring-shapedpermanent magnet 30 is fastened onto the center range 76 of the valvehousing 14 and is fixed axially by means of a ring-shaped supportingelement 78. Plunger coil 26 is disposed axially movably in an operatingair slot which is configured between the wall of the screening cage 77and the permanent magnet 30.

A captivating element 28 with whose aid the control slide valve 18 iscaptivated to the plunger coil 26 is fastened to the plunger coilsupporting 27. Captivating element 28 is comprised of a head 29 which inthe position of rest is in abutment from below against the main slidevalve 16. Beneath the plunger coil support 27 the screening cage 77defines a hydraulic chamber 31 which is filled with oil and which is inconnection with the storage tank 8 through laterally axially extendingducts 63.

The plunger coil 26 of the electromagnetic actuating unit requires to besupplied with electric current in order to safeguard the intendedfunction of the inventive two-tube shock absorber. The electromagneticactuating power which comes about will generate a downward movement ofthe control slide valve 18, so that a connection between the storagetank 8 and the interior space of the central tube 9 is created. Now,when the piston 3 moves upward in the traction stage, the pressureexisting in the power chamber 5 will become higher than the pressure inthe power chamber 6 disposed beneath the piston 3, whereas the firstnon-return valve 4 will remain closed. As a result, a displacement ofthe oil will take place by the hollow piston rod 2 out of the chamber 5into the interior space of the central tube 9 and through the open shockabsorber valve 10 into the annular chamber 44. From annular chamber 44,the oil flows through bores, shown as ducts 61, in the disc-shapedelement 38 of the second non-return valve 13 into the power chamber 6which is arranged beneath the piston 3, which is simultaneously refilledfrom the storage tank 8.

In the event of a movement of the piston 3 in downward direction in thethrust stage, the pressure existing within the power chamber 6 situatedbeneath the piston 3 will become higher than the pressure within theupper power chamber 5, so that the first non-return valve 4 will beopened. The volumetric stream exiting from the power chamber 6 isdivided, more precisely, into a first part-stream which flows throughthe open first non-return valve 4 directly into the power chamber 5positioned above the piston 3, and into a second part-stream which flowsthrough the hollow piston rod 2, through the central tube 9 and throughthe open shock absorber valve 10 into the storage tank 8.

In the operation of the inventive two-tube shock absorber, two operatingstatuses of the shock absorber valve 10 are distinguished. In the firstoperating status, which corresponds to a lower range of piston speed inwhich low pressures and small volumetric streams occur, the shockabsorber valve 10 works as a single-stage slide valve whereinelectromagnetic actuation determines the opening degree of the pilotcontrol stage 11.

In the second operating status, which corresponds to an upper range ofpiston speed in which more elevated pressures and larger volumetricstreams occur, the control slide valve 18 is positioned by the plungercoil 26 which is supplied with electric current, exactly as in the firstoperating status. The control slide valve 18 position is pre-establishedby an equilibrium between the force of the second compression spring 25,the actuating power of the plunger coil 26, the force resulting from thepressure differential between the pressure existing within the mainslide valve 16 and the pressure within the storage tank and acting onthe captivating element 28, and hydrodynamic flow forces occurring inthe range of the mouth of the control bores 17. When the control bores17 are closed, the pressure existing within the main slide valve 16equals the system pressure, so that the main stage 12 remains closed bythe force of the first compression spring 21. When the control bores 17are released, due to the flow of oil through the restricting bores 23 atthe main slide valve 16, a pressure differential will exist that acts onthe front face of the main slide valve 16 to release a force whichovercomes the spring force of the first compression spring 21. The forceresults in a movement of the main slide valve 16 in downward direction.In so doing, the control edge 15, and the flow-off bores 19 will bereleased. The movement of the main slide valve 16 mentioned above willresult in a partial closure of the control bores 17, so that thevolumetric stream flowing through the restricting bores 23 will decreaseand the pressure differential acting upon the main slide valve 16 willbe reduced.

This control process will continue until the spring force of the firstcompression spring 21 overcomes the force resulting from the pressuredifferential and will close the main stage 12. Due to this mechanism,the position of the main stage 12 is coupled hydraulically to theposition of the pilot control stage 11, the actuating energy needed forthe motion of the main slide valve 16 being taken from the flow whichhas to be throttled. In the upper range of piston speed, the shockabsorber valve 10 works as a two-stage pressure limiting valve. Theelectromagnetic actuation determines the level of the pressuredifferential at the main slide valve 16.

The continuous transition between these operating ranges may be modelledby the suitable dimensioning of the components and almost optional bandsof characteristic curves, which may be generated as characteristic ofthe shock absorber valve. In particular, a soft set-off of the valvecharacteristic curves in the lower operating range is rendered possiblein this way, which corresponds to a more comfortable setting of theshock absorber in case of its application in controllable vehicle shockabsorbers. In the upper operating range, the degressivity of thecharacteristic curves is essentially determined by the cross-sectionalarea of the captivating element 28 at its point of penetration into thevalve housing 14.

In the second embodiment of the invention which is illustrated in FIG.3, an intermediate position of the control slide valve 18 is shown inthe condition of rest of the shock absorber. In this position, thecontrol bores 17 are partially covered by the control slide valve 18.According to the invention, the intermediate position is reached by theaction of a third compression spring 60 which counteracts the secondcompression spring 25 prestressing the control slide valve 18 in theclosing direction. Third compression spring 60 is positioned in acylindrical recess 64 configured in the valve housing 14, coaxially withthe captivating element 28 in such a manner that it takes support at theplunger coil support 27.

In the third embodiment of the inventive two-tube shock absorberillustrated in FIG. 4, the actuation of the pilot control stage 11 takesplace by means of a proportional magnet 39 whose armature 40 isslidingly guided within a tubular coil support 65 and is connected bythe captivating element 28 to the control slide valve 18. In thisconfiguration, the main stage 12 is preferably furnished with a valvepiston 42 which is provided with two restricting bores 23 as well aswith a plurality of control bores 43. Valve piston 42 includes a conicalsealing surface 66 which interacts with a ring-shaped sealing seat 41configured at the disc-shaped element 38 of the second non-return valve13. Due to the use of the proportional magnet 39, a favorable weight perunit of power of the pilot control stage 11, i.e. a favorable ratiobetween the maximum magnetic force and the weight of the actuating unit,is achieved while the main stage 12, which is configured in the shape ofa seat valve, operates virtually free of leakage.

In the event of a failure, or of a faulty control action upon the shockabsorber valve 10 in the pressure stage to eliminate inadmissiblyelevated shock absorbing power levels, a non-controllable valve 32,preferably a non-return valve, is shown in FIG. 5 parallel to the mainstage 12, and to the pilot control stage 11. In the event of exceedingthe admissible system pressure, non-controllable valve 32 opens up aconnection between the power chamber 6 positioned beneath the valvepiston 3, on one hand, and the storage tank 8, on the other hand. Valve32 is, in this context, preferably positioned in the disc-shaped element38 of the second non-return valve 13 and is comprised of a ball 34,which is prestressed by means of a spring 35, and a ring-shaped sealingseat 33 which interacts with said ball 34. The connection is formed bybores 36, 37 in the disc-shaped element 38, and in the valve housing 14,respectively. The latter bore 37 ends in one of the flow-off bores 19.

In the embodiments of the invention illustrated in FIGS. 6 and 7, theactuating unit of the pilot control stage is formed by a plunger coil 26similar to the embodiment of FIGS. 1 and 2. Plunger coil interacts witha permanent magnet 30 located radially externally. The valve housing 14,presenting a two-part configuration, is furnished in its interior spacewith a ring-shaped plate 67 which is abutted, on one side, against thepermanent magnet 30. Valve housing 14 is formed, on the other side, witha central opening 68 which accommodates the plunger coil 26. In thisarrangement, the control slide valve 49 of the pilot control stage ispreferably comprised of a central cylindrical range of the plunger coilsupport 27 which is guided in an axially extending cylindrical wall 51of the valve housing 14 defining a hydraulic chamber 50 which is inconstant connection with the storage tank 8 and is formed with radialbores 52. Bores 52 interact with control bores 53 which are configuredin the control slide valve 49 and afford a link between the chamber 50and a chamber 54 which is configured in the valve housing 14 and whichis defined by a piston 47 sealedly and axially slidingly guided withinthe valve housing 14. The chamber 50 is defined at the top by ahorizontally extending wall 55 which closes the cylindrical wall 51 andwhich is provided with a stepped bore 56. Stepped bore 56 is in thiscontext preferably configured such that a section 57 of smaller diameterends up in the chamber 54, whereas a section 58 ending up in the chamber50 slidingly accommodates a cylindrical thrust member 59 which is inabutment against the control slide valve 49 from above.

At its upper end piston 47 mentioned before bears a circular valveclosing plate 45 which interacts with a sealing seat 48 configured inthe disc-shaped element 38. In addition, the piston 47 is formed with anaxially extending bore 46, which provides a connection between theinterior space of the central tube 9 and the hydraulic chamber 54. BothFIGS. 6 and 7 illustrate that the piston 47 jointly with the valveclosing plate 45 and the sealing seat 48 forms the main stage of theshock absorber valve mentioned in the foregoing text. In the presence ofan actuating power equaling zero, the thrust member 59 affords in thisembodiment a partial opening of the shock absorber valve, or of thelatter's pilot control stage.

The embodiment illustrated in FIG. 7 corresponds largely to the versionwhich has been described in conjunction with FIG. 6. However, in thecondition of rest the pilot control stage of this embodiment is in anintermediate position in which a partial overlapping of the openingranges in the control slide valve 49 and in the cylindrical wall 51takes place so that the shock absorber is partially open. In thisembodiment, the third compression spring 60 which has been mentioned inconnection with FIG. 3 is positioned between the horizontal wall 55 andthe control slide valve 49.

FIGS. 8 to 12 show five different embodiments of the electromagneticactuating unit which can be used in the aforementioned controllabletwo-tube shock absorber. In the second embodiment, illustrated in FIG.8, the coil support 72 of the plunger coil 71 is positioned outside themagnet frame 69. The connection between the coil support 72 and a coilform 74 bearing the winding of the plunger coil 71 is achieved by meansof guide pins 73. Guide pins 73 are passed through openings 75configured in the bottom of the magnet frame 69. As further shown inFIG. 8, the radially polarized ring-shaped permanent magnet 30 is fixedto the center portion of the magnet frame 69, approximately in itsmiddle, by means of two ring-shaped supporting elements 78. As a result,the magnetic flux of force generated by the permanent magnet 30 issubdivided into two part-fluxes 80 which are directed opposite eachother in the magnet frame 69, so that a symmetrical homogenous magneticfield of force will result in the operating-air slot which is defined bythe permanent magnet 30 and by the vertical wall of the magnet frame 69.

In the third embodiment of the inventive actuating unit, illustrated inFIG. 9, the electric coil 79 is unmovably positioned in the magnet frame69. The ring-shaped permanent magnet 30 is fixed to the cylindrical wallrange 82. Cylindrical wall range 82 defines the magnet frame 69 radiallyinternally and is axially slidingly guided in guides 81 made ofmagnetically non-conductive material. When the electric coil 79 issupplied with electric current, a force which is proportional to thecurrent intensity and direction will be generated at a plate 83 closingthe cylindrical wall range 82. The force results in a slide in the axialdirection of cylindrical wall range 82 together with the permanentmagnet 30.

In the fourth embodiment shown in FIG. 10, the radially polarizedpermanent magnet 84 is arranged centrically at the internal side of theradially external wall of the magnet frame 69 and is positioned by meansof two supporting elements 78. A coil support 86 for the plunger coil 85is guided in a manner similar to the embodiments according to FIG. 2 andFIG. 3, in guide slots 88 which are configured in the lower range of themagnet frame 69. Plunger coil 85 is radially encircled by the permanentmagnet 84. As a result, a more rational utilization of the magnetmaterial is rendered possible while the external dimensions remainunvaried.

In the fifth embodiment illustrated in FIG. 11, an improvement of theguidance of the plunger coil 26 in the cylindrical wall range 82 isshown. The cylindrical wall range 82 defines the radially internalportion of the magnet frame 69. The plunger coil support 27 of theplunger coil 26 radially encircles the permanent magnet 30. The coilsupport 27 is furnished with a central, cylindrically configured range87 which projects into the cylindrical wall range 82 of the magnet frame69. The coil support 27 external diameter corresponds to the internaldiameter of the wall range 82.

FIG. 12, finally, shows a sixth embodiment of the inventive actuatingunit in which the wall range 82 radially inwardly defining the magnetframe 69 is configured in the shape of a polygon. The polygon is shownin the shape of a hexagon 89. The magnetic field of force existing inthe magnet frame 69 is generated by a plurality of cuboid-shaped,axially magnetized partial magnets 90 which are fastened to the frontsurfaces of the hexagon 89. Alternatively, magnets 90 may be shaped ascircular discs, or as ring segments.

FIG. 13 shows a coil arrangement which may be used in the embodiments ofthe electromagnetic actuating unit according to the invention which havebeen described before. On the plate-shaped coil support 92 formed ofmagnetically non-conductive material, a single-layer winding 91 of metalribbon is disposed. Winding 91 is preferably comprised of a unilaterallyinsulated strip of copper foil or by a strip of aluminum foil which hassuperficially been oxidized. The layer of oxide will then form theelectrical insulation of one of the windings with respect to the other.The coil support 92 may preferably be furnished with a guide pin 93which is disposed in its center and which may, for example, serve forthe transmission of an actuating force to relevant components of theshock absorber valve. A coil arrangement of this kind may also be usedfor magnet systems according to FIG. 9 in which the electric coil isdisposed unmovably.

We claim:
 1. A two-tube shock absorber with controllable variable shockabsorbing power, comprising:a power cylinder having interior spacesubdivided into a first and a second power chambers by a piston, saidpiston being slidable by a piston rod; a storage tank partly filled withoil, said storage tank having a plurality of hydraulic connections whichafford a compensation of volume between said power chambers and saidstorage tank, a first and a second non-return valves, and a shockabsorber valve, said shock absorber valve providing the variation of thecross-sectional area of flow of one of said hydraulic connections, saidshock absorber valve forming a pilot control stage and a main stage,wherein in a lower range of piston speed the shock absorbing power isexclusively determined by the variable flow cross-sectional area of saidpilot control stage, said main stage remaining closed, and wherein in anupper range of piston speed the shock absorbing power is determined bythe variable flow cross-sectional areas both of said main stage and ofsaid pilot control stage.
 2. A two-tube shock absorber as claimed inclaim 1, wherein said main stage follows the position of said pilotcontrol stage in the upper range of piston speed.
 3. A two-tube shockabsorber as claimed in claim 2, wherein said main stage and said pilotcontrol stage are arranged so as to be separate from said secondnon-return valve in a valve housing, said main stage including a mainslide valve interacting with a control edge which is configured withinsaid valve housing and said pilot control stage including anelectromagnetically actuatable control slide valve interacting with aplurality of control bores configured in said main slide valve.
 4. Atwo-tube shock absorber as claimed in claim 3, wherein said control edgeis positioned in proximity to a plurality of flow-off bores configuredin said valve housing, said flow-off bores being in connection with saidstorage tank.
 5. A two-tube shock absorber as claimed in claim 4,wherein said main slide valve is configured open in said upper range,said main slide valve being furnished with a plurality of bores whichconnect said main slide valve with said flow-off bores.
 6. A two-tubeshock absorber as claimed in claim 3, wherein said main slide valve isprestressed in the closing direction of said main stage by means of afirst compression spring, said main slide valve being in abutmentagainst a stop in a condition of rest.
 7. A two-tube shock absorber asclaimed in claim 6, wherein said main slide valve includes an axialprojection, said axial projection abutting said stop.
 8. A two-tubeshock absorber as claimed in claim 3, wherein said main slide valve isprovided with a plurality of restricting bores which end up in ahydraulic chamber defined within said valve housing of said main slidevalve.
 9. A two-tube shock absorber as claimed in claim 3, wherein saidcontrol slide valve is guided in said main slide valve and is abuttedagainst said main slide valve in a condition of rest under theprestressing action provided by a second compression spring.
 10. Atwo-tube shock absorber as claimed in claim 3, wherein said pilotcontrol stage is actuatable electromagnetically by a plunger coil whichinteracts with a permanent magnet, and wherein said control slide valveis captivated to a plunger coil support by a captivating element, a headof said captivating element being in abutment against said main slidevalve in a condition of rest.
 11. A two-tube shock absorber as claimedin claim 10, wherein an actuating unit is comprised of said plunger coiland said permanent magnet, said permanent magnet positioned in ahydraulic chamber which is in connection with said storage tank, so thatthe end of said captivating element is fixed to said plunger coilsupport and is subject to the pressure existing within said storagetank.
 12. A controllable shock absorber as claimed in claim 3, whereinthe mode of functioning in the upper range of piston speed correspondsto the mode of functioning of a two-stage pressure limiting valve, theelectromagnetic actuation of said pilot control stage determining thelevel of pressure differential at said main slide valve.
 13. A two-tubeshock absorber as claimed in claim 1, wherein a non-controllable valveis provided parallel to one of said main stage and said pilot controlstage, said non-controllable valve being active exclusively in a thruststage of the shock absorber.
 14. A two-tube shock absorber as claimed inclaim 13, wherein said non-controllable valve is configured in the rangeof said second non-return valve, said non-controllable valve including aball, said ball interacting with a valve seat, said non-controllablevalve being prestressed by a spring.
 15. A two-tube shock absorber asclaimed in claim 13, wherein said non-controllable valve is arranged ina connection between said second chamber and said storage tank, saidmain stage and said pilot control stage being arranged in a valvehousing, with a plurality of flow-off bores configured in said valvehousing, wherein said connection is formed by a bore in a disc-shapedelement of said second non-return valve, and a bore in said valvehousing, said valve housing bore ending up in one of said flow-offbores.
 16. A two-tube shock absorber as claimed in claim 1, wherein saidmain stage includes a main slide valve and said pilot control stageincludes a control slide valve actuatable electromagnetically, theelectromagnetic actuation being achieved by a proportional magnet, saidmagnet having armature connected to said control slide valve by acaptivating element, a head of said captivating element being abuttedagainst said main slide valve in a condition of rest.
 17. A two-tubeshock absorber as claimed in claim 1, wherein said main stage and saidpilot control stage are arranged within a valve housing so as to beseparate from said second non-return valve, said main stage including avalve piston interacting with a sealing seat which is configured as adisc-shaped element of said second non-return valve, and said pilotcontrol stage including an electromagnetically actuatable control slidevalve interacting with a plurality of control bores configured in saidvalve piston.
 18. A two-tube shock absorber as claimed in claim 17,wherein said disc-shaped element and said valve housing define ahydraulic annular chamber into which said valve piston projects andwhich is in connection with said storage tank.
 19. A two-tube shockabsorber as claimed in claim 18, wherein said control bores connect withsaid annular chamber.
 20. A two-tube shock absorber as claimed in claim1, wherein said main stage and said pilot control stage are positionedwithin a valve housing so as to be separate from said second non-returnvalve, said main stage including a piston sealedly guided within saidvalve housing, said piston bearing a valve closing plate, said pistonhaving an axial bore, said main stage having a sealing seat shut off andreleased by said valve closing plate and which is configured as adisc-shaped element of said second non-return valve, said pilot controlstage including a cylindrical chamber configured in said valve housing,said pilot control stage accommodating a control slide valve elasticallyprestressed in the closing direction wherein an axially extendingcylindrical wall is provided with radial bores interacting with controlbores configured in said control slide valve to afford a hydraulic linkbetween said storage tank and a chamber, said chamber being defined bysaid piston within said valve housing.
 21. A two-tube shock absorber asclaimed in claim 20, wherein said pilot control stage is actuatableelectromagnetically by an actuating unit, said unit comprising saidcontrol slide valve being configured with a support member of a plungercoil.
 22. A two-tube shock absorber as claimed in claim 20, wherein ahorizontally extending wall is provided which defines said cylindricalchamber and which is formed with a stepped bore, said bore having asmall diameter section in said chamber defined by said piston withinsaid valve housing, and said bore having a larger diameter section whichaccommodates an axially slidable thrust member which interacts with thefront face of said control slide valve.
 23. A two-tube shock absorber asclaimed in claim 20, wherein a partial connection exists between saidchamber defined by said piston within said valve housing and saidstorage tank, said partial connection existing when said control slidevalve is in an intermediate position in a condition of rest.
 24. Atwo-tube shock absorber as claimed in claim 23, wherein saidintermediate position of said control slide valve is achieved by a thirdcompression spring which counteracts a second compression springprestressing said control slide valve.
 25. A controllable two-tube shockabsorber as claimed in claim 24, wherein said third compression springis clamped between a booster housing and a plunger coil support.
 26. Ashock absorber as claimed in claim 25, wherein said third compressionspring is positioned in a cylindrical recess in said valve housing. 27.A two-tube shock absorber as claimed in claim 20, wherein a thirdcompression spring is supported at a horizontal wall defining saidcylindrical chamber.
 28. A controllable shock absorber as claimed inclaim 1, wherein the mode of functioning in the lower range of pistonspeed corresponds to the mode of functioning of a single-stage slidevalve whose electromagnetic actuation determines the degree of openingof said pilot control stage.